Machines for preparing beverages have been developed, such as e.g. machines for preparing coffee, presenting the particularity that the liquid pressure to obtain the coffee is provided by systems alternative to the electric pressure pump, to obtain autonomous machines, in which the coffee can be distributed avoiding said pump and the maintenance and low power efficiency problems involved with its use.
The international patent application WO2010/113116 discloses a machine for preparing coffee of said kind, in which the system for supplying pressurized liquid comprises a first cylinder and plunger assembly provided with a chamber for housing water, the plunger of said chamber being able to accumulate, in an active position, potential energy from the compression force applied to a fluid provided in the chamber that houses the rod of said plunger. In said patent application, the potential energy accumulated by the plunger can be transmitted as pressure to the water inside said chamber, when it contacts said plunger, in its active position, with the water inside said chamber.
The system for supplying pressurized liquid disclosed in said patent has the advantage that it is an efficient system that permits to carry out a stable liquid distribution at a constant high pressure of 5-10 bars (pressure needed for the “espresso” coffee), optimising the electric consumption and the manufacturing and maintenance costs of the prior art machines for preparing beverages.
However, the pressurized system of said patent has some drawbacks. For example, pressure losses of the fluid chamber have been detected, and also possible contamination problems of the fluid inside the chamber that houses the liquid to be pressurized supplied.
To solve these problems, the enhancements have been focused on the sealing of the cylinder and plunger assembly that houses the fluid and the liquid. However, it has been observed that this enhancement makes the cost the manufacturing and the maintenance of the machines of the system more expensive and complicated.